Single photon detection has become possible since the invention of photomultiplier tubes (PMTs), which has found many applications in many different areas from then on. However, PMTs have two main handicaps: their high sensitivity to magnetic fields and their high prices.
Silicon photomultipliers (SiPMs) have been developed during recent years as a possible alternative to PMTs. Nowadays, the performances of SiPMs are fast approaching those of conventional PMTs. In addition, SiPMs have a lot of advantages over PMTs, such as compactness, low bias voltage operation, magnetic field insensitivity and fast timing response. The SiPMs can also take advantage of the highly developed silicon (Si) process technologies and the modern fabrication facilities for batch-processing in semiconductor industry, which guarantees the robustness and low fabrication costs of the devices. As a result of the excellent properties associated with SiPMs, SiPMs have found widespread applications in high-energy physics, fluorescence and luminescence decay measurements, single-molecule detection, laser ranging, nuclear medical imaging such as Positron Emission Tomography (PET), radiation detection for homeland security systems, and so on. The detection of photons in visible and ultraviolet regions is of special interest in recent years.
SiPMs have been investigated and developed for many years, and many ideas and technologies have been adopted to increase the fill factors of SiPMs. However, there remains challenges associated with conventional SiPMs, for example in terms of the dark current/dark count rate and/or the photon detection efficiency (PDE). For example, conventional SiPMs have high dark current, high crosstalk, and low fill factor (FF).
Furthermore, relatively complicated fabrication processes may be required to fabricate conventional SiPM devices, consequently resulting in a decrease in the yield and an increase in the fabrication cost. For some conventional SiPM devices, advanced wafer bonding or epitaxial technologies will be needed to realize the thick epitaxial layers. There are also issues of poor reproducibility of the conventional SiPM devices.
In addition, conventional SiPM devices may use polysilicon (poly-Si) resistors to realize current quenching. The related fabrication processes, such as deposition, doping, etching and contacting of poly-Si, will decrease the reproducibility of resistors, limit the yield, and result in increased fabrication cost. More importantly, the poly-Si strips need to be designed to be wide and long enough to guarantee the stability and reproducibility of the resistor, which accordingly reduce the fill factor of the device.